1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, and a process cartridge and an electrophotographic apparatus having such an electrophotographic photosensitive member.
2. Related Background Art
Conventionally, various systems such as an electrophotographic system, a thermal transfer system and an ink jet printing system have been employed for an image forming apparatus. Of those, an image forming apparatus employing the electrophotographic system (electrophotographic apparatus) is superior to those employing the other systems because it achieves a high speed, high image quality and quiet printing.
Furthermore, not only a monochrome electrophotographic apparatus but also a multiple color electrophotographic apparatus (a color electrophotographic apparatus) have recently become popular.
The color electrophotographic apparatus employs various well known systems such as a multiple transfer system, an intermediate transfer system or an inline system. Specifically, the multiple transfer system includes successively performing an exposure and a development for each color on a single electrophotographic photosensitive member, and successively transferring a toner image for each color to a transfer medium (e.g., paper) which is held on a transfer medium holding member (e.g., transfer drum), so as to form a color image. The intermediate transfer system includes successively performing an exposure and a development for each color on a single electrophotographic photosensitive member; successively primarily transferring a toner image for each color to an intermediate transfer member (e.g., an intermediate transfer drum, an intermediate transfer belt) (a primary transfer); and secondarily transferring in batch the primarily transferred image to a transfer medium (a secondary transfer), so as to form a color image. The inline system includes forming a toner image for each color in image forming portions for each color respectively, in which the image forming portions for each color are arranged in series and respectively have an electrophotographic photosensitive member, exposure means and development means, and successively transferring the toner image for each color to a transfer medium which is conveyed successively to the respective color image forming portions by a transfer medium convey member (e.g., transfer medium conveyor belt), so as to form a color image.
According to such a color electrophotographic apparatus, a plurality of transferring operations from an electrophotographic photosensitive member to a transfer medium or from an electrophotographic photosensitive member to an intermediate transfer member are required. Therefore, in the second or subsequent transferring operation, there exists toner for the color previously transferred on a portion to be printed between the electrophotographic photosensitive member and the transfer medium or between the electrophotographic photosensitive member and the intermediate transfer member. As a result, there arises a difference in transfer current between a portion where the toner exists and a portion where the toner does not exist. The transfer current difference causes a potential difference on the electrophotographic photosensitive member. The larger the transfer current, the more remarkable the tranfer current difference, which causes a faulty image such as a ghost image.
Furthermore, according to a color electrophotographic apparatus, the transfer current in many cases is set to be larger and larger as a new color image is overlapped on the previously transferred color image in order to avoid a so-called retransfer, i.e., a phenomenon in which the toner for the color previously transferred is carried back to the electrophotographic photosensitive member when the subsequent color image is transferred. Therefore, a faulty image due to the potential difference on the electrophotographic photosensitive member would be caused more easily as the more subsequent color image is transferred.
The large transfer current applies a high potential transfer bias to the electrophotographic photosensitive member, the intermediate transfer member or the transfer medium convey member. As a result, pin-hole leakage would be caused; durability of the electrophotographic photosensitive member would be deteriorated; and a material itself, which constitutes the electrophotographic photosensitive member, would be broken.
It is demanded for the whole electrophotographic apparatus to achieve sufficiently high transfer efficiency without causing large transfer current. Especially, higher transfer efficiency and smaller transfer current are eagerly required for a color electrophotographic apparatus as described above.
As means for improving transfer efficiency, techniques for improving a releasing property on the surface of an electrophotographic photosensitive member are known. Such techniques include incorporating silicone resin in the surface layer of the electrophotographic photosensitive member, or dispersing fluorine atom-containing resin particles (fluororesin powders) therein.
However, in the case of incorporating silicone resin in the surface layer of the electrophotographic photosensitive member, a sufficient releasing property necessary for transferring can not be obtained, while water repellency on the surface of the electrophotographic photosensitive member is somewhat improved. Furthermore, since silicone resin easily migrates to the surface of the layer, effect by the silicone resin is obtained only at the initial stage of use. More specifically, as the surface of the electrophotographic photosensitive member is worn out by a long-term use, the effective component is lost. As a result, the effect can not be maintained for a long period of time.
Compared to a technique using silicone resin, a technique for dispersing fluorine atom-containing resin particles in the surface layer of the electrophotographic photosensitive member is expected to provide an improved releasing property on the surface of the electrophotographic photosensitive member. Examples of the fluorine atom-containing resin particles include: tetrafluoroethylene resin; trifluorochloroethylene resin; tetrafluoroethylene hexafluoroethylenepropylene resin; vinyl fluoride resin; vinylidene fluoride resin; polydichlorodifluoroethylene resin; and a copolymer thereof (disclosed in Japanese Patent Application Nos. 2000-081715 and 2001-249481). Furthermore, Japanese Patent Application Nos. 2000-081715 and 2001-249481 disclose that dispersibility of the fluorine atom-containing resin particles in the surface layer of the electrophotographic photosensitive member is improved by incorporating diorganopolysiloxane having a specific chemical structure therein.
However, since the fluorine atom-containing resin particles in the surface layer of the electrophotographic photosensitive member scatter exposure light, a sharp electrostatic latent image is hardly obtained, which may cause some restriction on image quality.
It is demanded for the whole electrophotographic apparatus to maintain a satisfactory releasing property on the surface of the electrophotographic photosensitive member for a long-term use and to obtain a sharp electrostatic latent image. Especially, obtaining more satisfactory releasing property and a sharper electrostatic latent image is eagerly required for a color electrophotographic apparatus as described above in which a plurality of colors of toner images are formed and the images are overlapped so as to form a color image.